Electrically conductive device and the manufacture thereof



A ril 29, 1941. J. A. BECKER ETAL ELECTRICALLY CONDUCTIVE DEVICE AND THE MANUFACTURE THEREOF Filed Oct. 7, 1937 2 Sheets-Sheet l PUMP INLET L/GU/ZJ All? TRAP JABECKER INVENTORS. WHBRATM/N Wm QM April 29, 1941. J. A. BECKER ETAL 2,239,770

ELECTRICALLY CONDUCTIVE DEVICE AND THE MANUFACTURE THEREOF Filed Oct. 7, 1957 2 Sheets-Sheet 2 FIG. 2

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A TTORNEV Patented A r. 29,1941

ELECTRICALLY CONDUCTIVE DEVICE AND I THE MANIJ'FAC'IURE- THEREOF Joseph A. Becker, Mountain Lakes, Walter H. Brattain, New York, N.

N. 1., and

Y., assignors to Bell Telephone Laboratories, ted, New York, N. Y., a corporation of New York Application mm 1, 1931, SerlaiNo.167',714 Claims. (Cl. 175-368) This invention relates to electrically conductive devices and to the manufacture thereof. More particularly, it relates to rectifier units of the dry contact type and to methods offabricating such units.

Rectifier units of this type, of which the copper-copper oxide rectifier is illustrative, comprise generally an electrical conductor and a semi-conductor intimately Joined to each other. During use of such units, electrical connection must be established, of course, to both the conductor and semi-conductor. The nature of the connection to the semi-conductor, it has been found, materialiy affects the impedance and rectifying characteristics of the unit. It has been found particularly that the known connections to the semiconductor vary in resistance over periods of use and that similar connections to several units often resultin unlike characteristics, because of the variations in the resistances of the contact between the semi-conductor and the connection thereto and the introduction of a rectifying Junction acting in opposition to that of the Junction between the conductor and the semi-conductor of the rectifier unit. Furthermore, the formation of a fairly satisfactory connection to the semiconductor heretofore has involved the use of relatively high pressures which may affect the characteristics of the rectifier units deleteriously', entails manufacturing difficulties and unduly restricts the size and hence the capacity of the rectifier units.

One object of this invention is to establish an efficient, low resistance and substantially nonre'ctifying junction between a conductor and a semi-conductor.

Another object of this invention is to obtain a junction between a conductor and a semi-conductor which will have substantially constant electrical characteristics throughout an extended period.

A further object of this invention is to assure uniformity of electrical characteristics among a plurality of similar rectifierunits.

Another object of this invention is to obtain a high rectification ratio in dry contact rectifiers.

Still another object of this invention is to expedite and to simplify the manufacture of composite electrically conductive devices, such as dry contact rectifiers, in quantity and at relatively.

low cost.

In one illustrative embodiment of this invention a rectifier comprises a disc of conductive material, such for example as copper, a disc or layer of semi-conductive material, for example cuprous oxide, intimately Joined to the conductive disc. and a low resistance contact material intimately Joined to the semi-conductive disc or layer.

In accordance with one feature of this invention, a restricted portion of the semi-conductive element is subjected to an abrasive or other treatment to remove any insulating film thereon and to form a roughened surface to which contact material will adhere and thereafter a relatively large area con-tact element is formed upon this portion and homogeneously united thereto.

In accordance with another feature of this invention, the contact material is formed upon the semi-conductive element by vaporization of the material in vacuo and deposition thereof upon the previously treated and-roughened portion of the semi-conductive element.

The invention and the foregoing and other features thereof will be understood more clearly and fully from the following detailed description with reference to the accompanying drawings in which: v

Fig. 1 is a view partly in perspective and partly diagrammatic of apparatus which may be utilized for the deposition of the contact material upon the semi-conductive element of the rectifier unit, a portion of the housing being broken away to show the internal elements more clearly;

Fig. 2 is a plan view, with parts broken away, of a mask for mounting rectifier units during the processing of the semi-oonductiveelementand the formation of the contacts thereon;

Figs. 3 and 4 are enlarged sectional views showing details of the mask illustrated in Fig. 2, Fig. 4 showing also a rectifier uni-t mounted upon the mask; and

Fig. 5 is a perspective view partly in section of a typical rectifier unit constructed in accordance with this invention, the elements thereof being exaggerated as to thickness for the sake of clarity.

In the fabrication of rectifier devices in accordance with this invention, one surface of the conductive elements, which may be, for example, copper discs, are first oxidized by a suitable process. The black oxide is removed by chemical or other suitable means. The metal-metal oxide units are then placed in a mask and the exposed surface thereof air-blasted with a fine abrasive, such as 200 mesh silicon carbide. Other suitable abrasives such as steel grit or sand of the proper size may be used. The mask is such that the edges of the units are protected from the blasting 'and roughened also particles. A mask such as that shown inFigs. 2, 3 and 4 of the drawings may be used.

This mask, designated generally as 80, comprises a plate 8! having screw holes 32 therein and a perforated plate 33 which is secured to the first plate as by screws 34. Each perforation in the plate 33 has a portion 35 of a diameter and depth to receive a metal-metal oxide unit 50. one ofwhich is shown in Fig. 4 and another portion 35 of the diameter of the area to be coated with the contact metal. As many perforations as can be made without unduly weakening the plate are distributed over its area. A sheet of resilient material 31 having struck up spring portions. 3% is clamped between the plates SI and 38 so that a portion 38 is opposite each perforation. The units to be processed are inserted in the plate 33 and the sheet 31 is clamped over them by means of plate 3| and screws 34. Each spring portion 38 holds the corresponding unit firmly against a shoulder so that there will be no processing of the marginal portion of the units.

The air pressure during the blasting is much less than that ordinarily used in commercial sandblasting. In commercial sandblasting devices the air pressure is of the order of forty pounds'per square inch, whereas pressures of less than half this are successful in the present process.

Instead of treatment by blasting, the surface of the oxide may be mechanically worked by rubbing with an abrasive, such as sandpaper or by similar methods. The surface may be cleaned by chemical methods. For example, the red oxide surface of a copper-copper oxide unit may be chemically etched. Care must be taken in any event to avoid injury of the oxide layer and particularly the metal-metal oxide junction.

when blasting is employed the air pressure, size of the abrasive particles, distance from the nozzle and the time are so adjusted that a small portion of the red oxide is removed in small particles. vIf the impact of an individual blasting particle is so great as to remove a relatively large piece of oxide, the junction between the oxide and the mother copper may be injured. If this happens, the resistance in the high resistance direction is decreased. Furthermore, too coarse a roughening' may result in high resistance paths between the oxide and the metallic contact. Either of these effects decreases the rectification ratio. Similar precautions must be taken with rubbing or other mechanical working.

The amount of red oxide removed will, of

course, depend on the time of air-blasting. Ordinarily only a small amount of oxide is removed. This roughens the surface sufllciently and removes any film that may have been adsorbed preceding this treatment. The roughening of the surface increases the area of contact between the oxide and the metal and enables the formation of a more intimate mechanical bond between the two. If an adsorbed film of any appreciable thickness exists on the red oxide, it is important that it be removed in order to obtain an intimate electrical contact. Th airblasting as above described is an effective way of doing this. The air-blasting may be used to remove the black oxide which forms on top of the red oxide, in place of chemical treatment or other means. v

It is important that the surface to be coated with metal be kept clean from th time of cleaning until the metal has been deposited. Care should be exercised to protect the surface from all materials that might form an insulating mm thicker than about 5x10 centimeters. This is done in the vaporizing chamber by means more specifically described below.

After the cleaning operation the protected units are placed in a vaporizing apparatus. This apparatus may take the form shown in Fig. 1 in which I is a heavy metal base plate mounted on a suitable support. A bell Jar 2 is fitted to the plat with an airtight joint, which may be obtained by known means.

A plurality of insulated supports 3 project through the base plate I and are suitably sealed therein. These supports also act as conductors through which a heating current may be supplied to metal-holders 4, which are attached to the supports 3 and carry the metal to be vaporized. The carriers 4 may be'molybdenum, tungsten or tantalum wires coated with the metal to be vaporized, or may be boats or troughs of the same materials containing the metal.

The mask 30 is supported on the insulating tips In of the supports 3 with the exposed portion of the metal-metal oxide units toward the vaporizing means. The spacing between the mask 3 and the members 4 should be such that the units are not heated above 100 degrees to 150 degrees centigrade. The thickness of the film of metal deposited on the unit depends upon the amount i of metal supplied at the vaporizing means and metal-holders 4.

the distance of the mask therefrom.

Suitable pump means are connected to the pipe 5 through a liquid air trap. After the pump means has reduced the pressure toa low value, liquid air is introduced into the bulb or trap 6 inside the bell jar 2. This results in a further reduction of pressure. A particularly suitable pressure to be attained is of the order of 2x10 millimeters of mercury. Satisfactory contacts may be made, however. with a pressure ten times higher than this, but preferably the pressure is maintained below 1x 10- millimeters of mercury.

The pressure in the chamber is measured by an ionization gauge. This comprises a device 1 having a plurality of electrodes mounted in the chamber and connected with suitable sources of current, voltage and indicating devices by leads II. The device I may comprise the usual internal elements of a triode vacuum tube.

The metal is vaporized onto the units 50 by the passage of sufficient heating current through the The current is introduced through leading-in conductors I2 from a suitable source (not shown).

After the vaporization and deposition of the I contact material is completed, the current to the members 4 is shut off and the parts allowed to cool. After cooling, dry air is introduced through the pipe 8 which is normally closed by the valve 9. The mask is then taken out of the chamber. The units are removed from the mask and are ready to be tested and used.

It is desirable that the thickness of the metal film deposited be such that the electrical resistance in the direction parallel to the oxide surface is very low. Thisshould be of the order of .1 to .01 of an ohm between two electrodes, one centimeter wide and spaced one centimeter apart. With such low resistance contact need be made to this metal film at only a few points, and hence only small pressures need be used in assembling or mounting the units. Metal films whose thickness is about 10- centimeters or about .1 mil are satisfactory. For some uses, thinner films will be adequate.

It appears that the presence of small amounts of permanent gases in the vaporization chamber is not deleterious provided, of course, that these permanent gases do not chemically combine with the metal which is vaporized.

While the presence of gases is not particularly deleterious, the presence of vapors which might form adsorbed films on the oxide is very objectionable. The efiect of such adsorbed film is to greatly increase the contact resistance between the metal and the oxide and to produce a rectifying contact at the outer junction which opposes the rectification at the inner junction. The vapors of oils and greases, therefore, should be excluded or removed from the vaporizing chamber. This may be accomplished by the use of the liquid air trap in the vaporization chamber.

The contact resistance between the metal and the oxide preferably should be about .5 ohm per square centimeter or less. In semi-conductor and metal contact combinations constructed in accordance with this invention, the contact resistance per unit area attained is less than one-five hundredth of the specific resistance of the semiconductor.

Various means may be used in order to vaporize the contact metal. In addition to those alfrom a suitable crucible which may beheated by electron bombardment, by high frequency induction, or by some other suitablemeans.

Although in the preceding discussion particular reference has been made to the formation of contacts upon cuprous oxide, other semi-conductors may be coated with various metallic films, in accordance with this invention. For example, iron oxide,'galena, silicon carbide and selenium may be joined in intimate electrical contact with silver, gold, aluminum, zinc, cadmium or platinum. Combinations of two contact metals, such as silver and copper, have also been used successfully. Other materials. of course, may be used also. Of course, some metals are more dimcult to vaporize than others. In view of the number of different metals available, the use to be utilized and composite electrically conductive bodies formed without departing from the scope and spirit of this invention as defined in the appended claims.

What is claimed is:

1. The method of applying a metallic contact to a semi-conductor which comprises cleaning a surface thereof of any adsorbed layer of insulation, by removing the portion adjacent the surface containing said layer, placing the semi-conductor in a vaporizing chamber, reducing the pressure in said chamber by pumping and then further reducing the pressure and removing materials that might form an insulating film on the cleaned surface by the introduction of liquid air into a trap within said chamber, and vaporizing a metallic ready indicated, the metal may also be vaporized 1 which the finished product is to be put may dictate the ones to beemployed. For some of the j more common rectifier uses, silver appears to be particularly suitable.

Since only small pressures are required. in the assembly of rectifier units constructed in accordance with this invention, the need for a central hole as now used in the washers" no longer exists. The ratio'of periphery to area, therefore, can be reduced. Since the current at low voltages and the current in reverse direction is usuall proportional to the periphery, it will be appreciated that a higher rectification ratio can be obtained for devices constructed in accordance with this invention.

The small contact pressures required also allow the use of units having a much greater area than is practically possible with the older type contacts. It is possible with rectifiers having contacts formed as described hereinabove to use one, large area, unit in place of a plurality of the smaller units as previously employed.

Although the in ention has been described with respect to a particular illustrative embodiment, it is to beunderstood that it is not limited thereby, that various modification may be made therein, and that a variety of specific processes may surface while vaporizing a layer of metal thereon.

3. The method of applying a metallic contact to a semi-conductor which comprises cleaning a surface of said semi-conductor to remove any film of insulating material, placing a semi-conductor in a chamber, reducing the pressure in said chamber to a low value, further reducing the pressure and removing any vapors that might form an insulating film on the clean surface by applying liquid air to relatively large absorbing surfaces located in the chamber, and vaporizing a metal onto the cleaned surface.

4. The method of applying a metallic contact layer to the semi-conductor surface of a rectifier unit which comprises placing the unit in a mask which covers a small portion of the unit adjacent its edge, subjecting the exposed portion of the surface of the semi-conductor to a blast of silicon carbide, at a low blasting pressure, placing the mask and unit in a chamber, evacuating the chamber to a high degree, and depositing a metallic layer on the exposed portion.

5. The. method of applying a silver contact to the cuprous oxide layer of a copper-cuprous oxide rectifier unit. which comprises placing the umt in a mask for protecting the oxide surface adjacent the edge from further processing, cleaningthe exposed surface of oxide to remove any insulating film by applying a blast of silicon carbide at a pressure of about 20 pounds per square inch to it, maintaining the cleanliness of said surface and transferring the mask and unit to a vaporizing chamber containing a metal-carrier loaded with silver, placing" the mask in the chamber so that the oxide surface of the unit is toward the metal-carrier and spaced therefrom a distance such that the unit will not attain a temperature higher than degrees centigrade durlng vaporization, evacuating the chamber first by pumping and then by the introduction of liquid air into a trap in the chamber. to remove the vapors or gases that might form an insulating r film on the oxide surface, the final pressure being in the order of 2x 10- millimeters of mercury,

and vaporizing the silver onto the oxide surface ina thin layer by the application of heat to the metal-carrier.

' JOSEPH A. BECKER.

WALTER H. BRA'I'IAIN. 

